Filtered electro-explosive device

ABSTRACT

A pyrotechnic device includes a high pass, lossy, ceramic filter on the connector pin of the device. The pyrotechnic device includes a bridging wire between the connector pin and an outer metallic shield. Electrical current passing through the bridging wire detonates an explosive charge. The lossy filter shunts high frequency electrical noise away from the bridging wire thereby preventing accidental detonation of the charge.

Unite tates Patent 91 Hudson, Jr. et a1.

[11] 3,7353% i 1 May 29, 1973 v 1 FILTERED ELECTRO-EXPLOSIVE DEVICE [75] Inventors: William Jeffrey Hudson, Jr., Hummelstown; William Baird Fritz, Hershey, both of Pa.

[73] Assignee: AMP Incorporated, Harrisburg, Pa.

[22] Filed: July 15, 1971 [21] Appl. No.: 162,863

[52] US. Cl. ..102/28 R 511 Km. c1. ..F42b 19/12 [58] Field of Search ..l02/28 R, 28 EB [56] References Cited UNITED STATES PATENTS 3,351,012 11/1967 Wilson ..l02/28EB 3,572,247 3/1971 Warshall 102/28 R Primary ExaminerVerlin R. Pendegrass Att0rney-Wi11iam J. Keating, Ronald D. G'refe and Gerald K. Kita et a1.

[ 5 7 ABSTRACT A pyrotechnic device includes a high pass, lossy, ceramic filter on the connector pin of the device. The pyrotechnic device includes a bridging wire between the connector pin and an outer metallic shield. Electrical current passing through the bridging wire detonates an explosive charge. The lossy filter shunts high frequency electrical noise away from the bridging wire thereby preventing accidental detonation of the charge.

2 Claims, 2 Drawing Figures FILTERED ELECTRO-EXPLOSIVE DEVICE BACKGROUND OF THE INVENTION This invention relates to pyrotechnic devices and more particularly to a ceramic filter on the pin of the pyrotechnic device to prevent accidental detonation of the device caused by stray high frequency noise.

One type of commonly used pyrotechnic device includes an electrical detonator having a bridging wire which extends between a connector pin and an outer shell. An electrical voltage applied to the pin causes current to flow through the bridging wire. This explodes the bridging wire thereby igniting the explosive charge. Such devices are referred to as electroexplosive devices.

Such devices have always been beset with the problem of accidental detonation from high frequency electrical noise. The bridging wire acts as an antenna which picks up stray electro-magnetic radiation for example, such noise may be caused by radio transmission in the area.

Recently, RF filters have been extensively used in electrical circuits to suppress stray radio frequency noise. Filters of the type shown in US. Pat. No. 3,275,953 Coda et al. have been used as feed through filters or on connector pins.

RELATED APPLICATIONS Ser. No. 883,501, filed Dec. 9, 1969, to William B. Fritz discloses a ceramic filter which is particularly suitable for use in this invention.

SUMMARY OF THE INVENTION In accordance with this invention, accidental detonation of electro-explosive devices by high frequency noise is prevented by a tubular ceramic filter on the connector pin on the device.

In accordance with further aspects of this invention, the filter is a compact, low cost device including an extruded tube of ferrite with deposited barium titanate forming a dielectric layer thereon. A metallic plating on the filter has gaps near the extremities to electrically isolate an inner conductive surface and an outer conductive surface.

The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description, appended claims and drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows the pyrotechnic device of this invention; and

FIG. 2 shows the equivalent electrical circuit of the device.

DESCRIPTION OF A PARTICULAR EMBODIMENT In FIG. 1 the pyrotechnic device includes a powder can 11 holding an explosive charge (not shown) at the bottom thereof. A detonator includes an outer metallic shell 12 and a metallic connector pin 13. A bridging wire 14 is connected between the outer shell 12 and the pin 13. The pin 13 typically has a nickel cover and is gold plated. The outer shell 12 is stainless steel. The bridging wire 14 normally has a resistance of approximately 2 ohms.

A receptacle 15 in a socket 16 supplies DC current to the detonator. The circuit is through the inner conductor of a cable, which is shown diagrammatically at 26 in FIG. 2, through the receptacle 15, the pin 13, the bridging wire 14, the outer shell 12 and the top portion of the powder can, which is electrically connected to the shell of the socket 16, and the casing on the cable which is common, or ground, potential. When current is supplied through this circuit the bridging wire 14 explodes thereby igniting the explosive charge in the can.

In order to positively and reliably prevent high frequency noise energy from detonating the charge, a tubular filter 17 is positioned on the pin.

The tubular filter is of the type including a substrate which, in a preferred embodiment, is the extruded tube of ferrite 18. A dielectric layer 19 is a deposited coating of barium titanate.

A metal plating forms an inner conductive surface 20 and an outer conductive surface 21. Gaps in the metal plating at 22 and 23 electrically isolate the inner conductive surface 20 from the outer conductive surface 21. The inner conductive surface 20 is in electrical contact with the pin 13 and the outer conductive surface 21 is in electrical contact with the outer shell 12. The filter 17 shunts high frequency electrical energy to the outer shell and prevents it from appearing across the bridging wire 13 which otherwise well might detonate the charge.

The equivalent electrical circuit of the filtered pyrotechnic device is shown in FIG. 2. A source of DC voltage represented by the battery 24 is connected through the detonator switch 25 to the cable 26, with the cable being connected through the socket 16 to pin 13 at the point 27. The equivalent circuit between point 27 and the bridging wire 14 includes an inductance 28 and a resistance 29 established by the pin 13 and the ferrite tube 18. The capacitors 30 and 31 represent the capacitive path between the pin and the outer conductive surface 21 of the filter. The impedance of the outer conductive surface 21 has been subdivided into three different series paths including inductors 32, 33 and 34 and resistors 35, 36 and 37. Two ground return paths including the inductor 38, resistor 39, inductor'40 and resistor 41 complete the equivalent circuit.

While FIG. 2 shows the impedances lumped into two ground return paths, actually the ground return paths are distributed along the entire length of the outer shell 12. The result is an electrical filter which very effectively shunts high frequency noise energy to ground. Only the DC current from source 24 passes through the bridging wire 14. In this manner, a very effective, compact and low cost filter for preventing accidental detonation of the pyrotechnic device is provided.

While a particular embodiment of the invention has been shown and described, it will be understood that various modifications are within the scope of this invention.

What is claimed is:

1. A pyrotechnic device comprising:

a powder can,

an explosive charge at the bottom of said can,

a detonator at the top of said can including:

an outer metallic shell,

an inner metallic pin,

a bridging wire between said outer shell and said pin for detonating said charge when electrical current passes therethrough, and

a tubular filter including a substrate of ferrite, a dielectric layer directly on the outer surface of the substrate, and extending about said dielectric metallic plating being in contact with said shell so material, about the inner surface of said ferrite that said filter shunts high frequency electrical tube and about the ends of the filter, said metallic energy across said bridging wire without detonatplating having gaps near both extremities of said ing said charge. tube to electrically isolate the metallic plating on 5 2. The pyrotechnic device recited in claim 1 wherein the outside of said filter from the plating on the said dielectric is barium titanate coated on said subinner surface of said tube, the inner metallic platstrate. ing being in contact with said pin and the outer 

1. A pyrotechnic device comprising: a powder can, an explosive charge at the bottom of said can, a detonator at the top of said can including: an outer metallic shell, an inner metallic pin, a bridging wire between said outer shell and said pin for detonating said charge when electrical current passes therethrough, and a tubular filter including a substrate of ferrite, a dielectric layer directly on the outer surface of the substrate, and extending about said dielectric material, about the inner surface of said ferrite tube and about the ends of the filter, said metallic plating having gaps near both extremities of said tube to electrically isolate the metallic plating on the outside of said filter from the plating on the inner surface of said tube, the inner metallic plating being in contact with said pin and the outer metallic plating being in contact with said shell so that said filter shunts high frequency electrical energy across said bridging wire without detonating said charge.
 2. The pyrotechnic device recited in claim 1 wherein said dielectric is barium titanate coated on said substrate. 